The present invention relates to a novel hydrophobic multicomponent catalyst useful in the direct oxidation of hydrogen to hydrogen peroxide and to method for the preparation of such catalyst. More particularly, this invention relates to a novel hydrophobic multicomponent catalyst comprising a hydrophobic polymer membrane deposited on a Pd containing acidic catalyst, useful for the direct oxidation of hydrogen by oxygen to hydrogen peroxide, and a method for the preparation thereof. The present invention also envisages direct oxidation of hydrogen to hydrogen peroxide in the presence of the novel catalyst of the present invention.
The hydrophobic catalyst of the invention has a great potential utility in the chemical and petrochemical industries for the production of hydrogen peroxide by direct oxidation of hydrogen by oxygen to hydrogen peroxide in an environmentally clean manner.
U.S. Pat. No. 1,108,752 of Henkel et al discloses the use of palladium for promoting the formation of hydrogen peroxide and water from a mixture of hydrogen and oxygen. Since then, there are numerous disclosures of palladium containing catalysts useful for the direct oxidation of hydrogen by oxygen to hydrogen peroxide.
U.S. Pat. No. 4,832,938 of Grosser et al discloses a Ptxe2x80x94Pd bimetallic catalyst supported on a carbon, silica or alumina support for making hydrogen peroxide from direct combination of hydrogen and oxygen in an aqueous reaction medium. German Patent Ger. Offen. DE 4,127,918 of Lueckoff et al discloses a supported palladium gold catalyst for the manufacture of hydrogen peroxide from hydrogen and aqueous medium, the catalyst containing 5-95 weight % of Au and supported on carbon. A number of platinum group metal containing catalysts: (1) Pt-Group metal on high surface area support, such as carbon, silica or alumina (Ref. U.S. Pat. No. 5,169,618); (2) Pt-Group catalyst on solid acid carrier (Ref. EP application No. 504,741 A1); (3) Pt-Group element supported on Nbxe2x80x94or Ta oxide (Ref. WO 9,412,428 A1); (4) Sn-modified Pt-Group metals supported on catalyst carriers (ref. EP Application No. 621,235 A1); (5) Pt-Group metal catalyst supported on hydrophilic support (Ref. U.S. Pat. No. 5,399,334) for the oxidation of hydrogen to hydrogen peroxide are known in the prior art.
The above-mentioned Pd- of Pt-Group metal containing catalysts are hydrophilic in nature, and hence the aqueous reaction medium used in the oxidation of hydrogen to hydrogen peroxide over these catalysts is in close contact with the catalyst surface. When the above-mentioned catalysts are used, the selectivity for hydrogen peroxide is drastically reduced because of the close contact between the catalyst and the reaction medium. The hydrogen peroxide, which is formed by the reaction between hydrogen and oxygen on the catalyst and then absorbed in the reaction medium due to the high affinity between hydrogen peroxide and water, is readsorbed on the catalyst from the reaction medium and converted to water and oxygen. Fu et al also disclose that Pd catalysts supported on hydrophobic support are selective towards hydrogen peroxide formation in the oxidation of hydrogen [Ref. Fu et al., Stud. Surf. Sci. Catal., 72 (1992) 33-41].
A few Pt-group or Group VIII metal catalysts deposited on hydrophobic support, useful for the oxidation of hydrogen to hydrogen peroxide are also known in the prior art.
Japanese Patent Jpn. Kokai Tokyo Koho JP 01133909 A2 of Kyora discloses a Pt-Group metal catalyst carried on a hydrophobic support such as porous and hydrophobic Teflon. EP 3,660,419 A1 of Chuang discloses a Group VIII metal catalyst deposited on a hydrophobic support for the manufacture of hydrogen peroxide by reacting hydrogen with oxygen in an aqueous medium. Intl. Appl. WO 9314025 A1 of Chuang discloses a Group VIII metal on a partially hydrophobic and partially hydrophilic support, such as Pd on fluorinated carbon, as a catalyst for the oxidation of hydrogen to hydrogen peroxide.
Although the hydrophobic support used in these catalysts provides some hydrophobic character to the Pdxe2x80x94or Group VIII metal catalysts, they also suffer from various disadvantages and limitations. It is difficult to deposit catalytically active components from aqueous solution on a hydrophobic support as there is no wetting of the surface of the hydrophobic support by the aqueous solution. Another disadvantage is that hydrophobic supports such as Teflon or other hydrophobic polymer supports are thermally unstable at the calcination temperatures that are normally employed for decomposing the precursor compounds of catalytically active components of the catalyst. Yet another disadvantage is that the hydrophobic character of the support is lost at least partially, if not completely due to the deposition of hydrophilic catalytically active components on the hydrophobic support.
Apart from the above mentioned disadvantages and/or limitations, the prior art catalysts with or without hydrophobic support are employed in the oxidation of hydrogen by oxygen to hydrogen peroxide at a pressure much above the atmospheric pressure. At high pressure, the explosion hazards for the reaction between hydrogen and oxygen are higher.
Thus, there is a need for developing a new catalyst that is active in the direct oxidation of hydrogen to hydrogen peroxide even at atmospheric pressure and also has a hydrophobic character such that the selectivity for the formation of hydrogen peroxide by the reaction of hydrogen and oxygen in an aqueous medium is high.
It is an object of the invention to provide a novel hydrophobic catalyst useful for the direct oxidation of hydrogen by oxygen to hydrogen peroxide with high selectivity.
It is another object of the invention to provide a novel hydrophobic catalyst useful for the direct oxidation of hydrogen by oxygen to hydrogen peroxide, which has high selectivity even at atmospheric pressure and in an aqueous medium.
It is another object of the present invention to provide a process for the preparation of such novel hydrophobic catalyst for the direct oxidation of hydrogen by oxygen to hydrogen peroxide with high selectivity even at atmospheric pressure and in an aqueous medium.
It is a further object of the invention to provide a novel hydrophobic catalyst comprising PdO in a highly acidic environment created by a solid acid or a solid super acid such that the catalyst shows both high activity and high selectivity in the direct oxidation of hydrogen to hydrogen peroxide.
It is yet another object of the invention to provide a novel hydrophobic multicomponent catalyst that is useful for the direct oxidation of hydrogen by oxygen even at atmospheric pressure and shows high hydrogen conversion activity and high selectivity for hydrogen peroxide formation.
These and other objects of the invention are accomplished by providing a novel hydrophobic multicomponent catalyst comprising a hydrophobic polymer membrane deposited on a highly acidic Pd containing catalyst, and a process for the preparation thereof.
Accordingly, the present invention relates to a novel hydrophobic multicomponent catalyst useful in the direct oxidation of hydrogen by oxygen to hydrogen peroxide, said catalyst being of the formula:
R(a)/AxByPdOz(b)/X(c)/MOn(d)/N
wherein: R is a hydrophobic polymer, which forms a hydrophobic polymer membrane permeable to hydrogen, oxygen and hydrogen peroxide vapors; A is a metallic element selected from a group consisting of Ag, Au, Cu, Fe, Cd, Zn, Sn or mixtures thereof; B is a noble metal element other than palladium and selected from the group consisting of Ru, Pt, Rh, Ir, Os, or a mixture thereof; Pd is palladium element; O is oxygen element; X is a halogen element selected from the group consisting of F, Cl, Br, I or a mixture thereof; M is N an element selected from S, P, Mo, W, Ce, Sn, Th, or a mixture thereof; N is a catalytic porous solid, optionally supported on a conventional catalyst carrier; x is A/Pd mole ratio in the range of 1 to about 1; y is a B/Pd mole ratio in the range of 1 to about 0.5; z is the number of oxygen atoms needed to fulfill the valence requirement of AxByPd; n is the number of oxygen atoms required to fulfill the valence requirement of M; d is a weight percent loading of M deposited as MOn on the catalytic porous solid N, in the range of about 0.2 wt % to about 20 wt %, c is the weight percent loading of halogen X deposited on MOn(d)/N in the range of about 0.2 wt % to about 20 wt %; b is the weight percent loading of AxByPd on X(c)/MOn(d)/N in the range of about 0.1 wt % to about 20 wt %; a is the weight percent loading of the hydrophobic polymer R deposited on AxByPdOz(b)/X(c)/MOn(d)/N in the range of about 0.01 wt % to about 10 wt %.
The present invention also provides a process for the preparation of said novel hydrophobic multicomponent catalyst, said process comprising the steps of:
i. depositing MOn on the surface of a catalytic porous solid N, optionally deposited on a conventional catalyst carrier, by impregnating or coating N with a compound of M, wherein M is an element selected from the group consisting of S, Mo, W, Ce, Sn, or a mixture thereof, which on decomposition or calcination converts to the oxide form in quantity sufficient to obtain a weight percent loading of M on N in the range of about 0.02 wt % to about 20 wt %, subsequently drying the resulting wet mass and then calcining the dried mass in air, inert gas or under vacuum at a temperature in the range of about 400xc2x0 C. to about 800xc2x0 C. for a period in the range of about 0.1 h to about 10 h;
ii. halogenating the mass obtained in step (i) by impregnating it with one or more halogen containing compounds represented by the formula ED, wherein D is an anion selected from the group consisting of Fxe2x88x92, Clxe2x88x92, Brxe2x88x92, Ixe2x88x92 and (HF2)xe2x88x92, and E is a cation selected from the group consisting of NH4+ and H+, in a quantity sufficient to obtain a loading of halogen X on the mass obtained from step (i) in the range of about 0.02 wt % to about 20 wt % and subsequently drying the resulting wet mass and then calcining the dried mass in air, inert gas or under vacuum at a temperature in the range of about 300xc2x0 C. to about 600xc2x0 C. for a period in the range of about 0.2 h to about 20 h;
iii. depositing AxByPdOz on the surface of the halogenated mass, obtained in step (ii) by impregnating or coating it with compounds of A, B, and Pd wherein A is a metallic element selected from the group consisting of Ag, Au, Cu, Fe, Cd, Zn, Sn or mixtures thereof, B is a noble metal element selected from the group consisting of Ru, Pt, Rh, Ir, Os, or a mixture thereof, Pd is palladium element, which on decomposition or calcination converts to their oxide form, with A/Pd and B/Pd mole ratios being in the range of 0 to about 1 and 0 to 0.5 respectively, and in quantities sufficient to obtain a loading of AxByPd on the mass obtained in step (ii) in the range of about 0.1 wt % to about 20 wt % and subsequently drying the resulting wet mass and then calcining the dried mass at a temperature in the range of about 350xc2x0 C. to about 650xc2x0 C. in the presence of air or oxygen for a period in the range of about 0.2 h to about 20 h; and
iv. finally depositing a hydrophobic polymer membrane, which is permeable to hydrogen and oxygen gases and hydrogen peroxide vapors on the surface of the catalytic mass obtained in step (iii) by impregnating a hydrophobic polymer, with or without crosslinking agent, from its solution in an organic solvent in quantities sufficient to obtain a loading of hydrophobic polymer on the catalytic mass in the range of about 0.01 wt % to about 10 wt % and subsequently removing the solvent from the polymer impregnated catalytic mass under vacuum at a temperature below 100xc2x0 C. and then heating the solventxe2x80x94free mass in air or oxygen at a temperature in the range of about 40xc2x0 C. to about 250xc2x0 C. for a period in the range of about 0.01 h to about 10 h.
The catalytic porous solid used in the catalyst preparation process of the invention is selected from the group consisting of xcex3- or xcex7-alumina, silica, silica-alumina, amorphous zirconium hydroxide, zirconium oxide, thorium oxide, uranium oxide, rare earth oxide, titanium oxide, niobium oxide, tantalum oxide, yttrium oxide, gallium oxide, indium oxide, H+ form pentasil zeolites containing 5-membered oxygen rings and having the structures ZSM-5, ZSM-11 or ZSM-8, H mordenite zeolite, ultra stable HY zeolite or dealuminated HY zeolite, silicalite-I (high silica ZSM-5), silicalite II (high silica ZSM-11), high alumina MCM-41 zeolite, high silica MCM-41 zeolite with or without grafting by Al, Ga or transition element, an activated carbon or a mixture of two or more thereof. All these catalytic porous solids are well known in the prior art. The catalytic porous solid is optionally supported on a conventional catalyst carrier, such as monolith catalyst carriers, low surface area (1 less than m2sxe2x88x921), macroporous (pore size: above 20 nm), mesoporous (pore size: 1-20 nm) and catalyst carriers in a form of pellets or granules of different sizes and shapes and/or mesoporous (pore size: 1-20 nm) high silica zeolites such as high silica MCM-41, by depositing precursor compound of the catalytic porous solid on the catalyst support or carrier by impregnation, coating or precipitation technique followed by drying and calcination at 250xc2x0 C. to 800xc2x0 C. The impregnation, coating and precipitation techniques are well known in the art.
Examples of the compounds of S, Mo, W, Ce, Sn, and P elements are as follows: the compounds of S are sulphuric acid and ammonium sulphate; the compounds of P are phosphoric acid and ammonium phosphates; the compounds of Mo are ammonium molybdate and molybdenum oxide; the compounds of W are ammonium metatungstate, tungsten oxide and tungstic acid; the compounds of Ce are cerium (III) nitrate, cerium(III) acetate, cerium(III) hydroxide, ammonium cerium(IV) nitrate and cerium(IV) oxide; and the compounds of Sn are tin(II) nitrate, tin(II) acetate and tin(II) oxide.
Examples of the compounds of the metallic elements, Ag Au, Cu, Fe, Cd and Zn and noble metal elements Pd, Pt, Ru, Rh, Ir and Os are as follows: the compounds of the metallic elements are their nitrates, acetates, chlorides, hydroxides, and oxides; the compounds of the noble metal elements are their nitrates, acetates, chlorides, ammonium salts, such as ammonium hexa chloro-palladate(IV), or -platinate (IV) or -osmate (IV) or -rhodate(III) or -ruthenate(IV) or -iridate(IV), chloro acids (for example, chloroplatinic acid, H2PtCl6), and the like.
The hydrophobic polymer used in step (iv) of the process of the invention is selected from polyfluorocarbons, polysulfones, polysiloxanes or silicon rubbers, polysulphide rubbers, and the like.
The halogenation or halidation of the mass obtained from step (i) of the process can also be done by contacting the mass with gaseous hydrogen halide or with the vapors of halohydrocarbon(s) at 100xc2x0 C.-500xc2x0 C. for a period sufficient to achieve the required weight percent loading of halogens on the mass. However, the method based on the impregnation of halogen containing compounds, as discussed step (ii) of the process of the invention is preferable.
The role of the catalytic porous solid N is to provide a support and also a highly acidic environment for the noble and other metal oxides in the catalyst, after modification of its surface acidity in the first two steps of the process. The catalytic porous solid is optionally supported on a conventional support which acts to provide a mechanically strong and/or thermally stable matrix for the catalytic porous solid, to increase the dispersion and hence the surface area of the catalytic porous solid, and also to reduce the pressure drop across the catalyst bed, particularly for fixed bed operation of the catalyst. The role of MOn that is incorporated in the catalyst in step (i) of the process, is to increase both the number of surface acid sites and the acid strength of the catalytic porous solid used in the catalyst preparation. For example, catalytic porous solids such as alumina, zirconia, titania, silica and tin oxide on their modification by SO4, WO3 or MoO3 are transformed into super acids.
The halogen elements X incorporated into the catalyst in step (ii) of the process greatly increase the surface acidity and/or change the nature of the surface acidity of the catalytic porous solid. For example, alumina, which contains only Lewis acid sites, on its modification by fluorine or chlorine containing compounds, is transformed into a protonic solid acid. Palladium oxide that is incorporated into the catalyst in step (iii) of the process provides catalytically active sites that are more selective in the acidic environment for the formation of hydrogen peroxide, for the direct oxidation of hydrogen by oxygen to hydrogen peroxide. The oxides of metallic elements A and noble metals other than Pd, designated as B, that are also incorporated into the catalyst along with palladium oxide in step (iii) of the process increase the hydrogen conversion activity of the catalyst in the direct oxidation of hydrogen by oxygen to hydrogen peroxide by acting synergistically. The hydrophobic polymer R incorporated into the catalyst in step (iv) of the process provides a hydrophobic character to the catalyst by forming a thin film or layer of hydrophobic polymer membrane on the catalyst that is permeable to oxygen, hydrogen and hydrogen peroxide vapors but not to liquid water. This avoids the direct contact between the active sites present on the catalyst and the aqueous medium thus drastically increasing the selectivity for hydrogen peroxide formation in the direct oxidation of hydrogen by oxygen to hydrogen peroxide in an aqueous medium.
In view of the above-mentioned roles of the various components of the catalyst, all the components and the steps of incorporation thereof into the catalyst are critical for achieving high hydrogen conversion with high hydrogen peroxide selectivity in the direct oxidation of hydrogen by oxygen to hydrogen peroxide in an aqueous medium.
In the catalyst of the invention and its preparation process, the preferred catalytic porous solid N, is an acidic porous solid selected from a group consisting of xcex3- or xcex7-alumina, silica-alumina, gallium oxide, cerium oxide, amorphous zirconia or zirconium hydroxide, thorium oxide, H-ZSM5 zeolite, H-ZSM11 zeolite, H-ZSM8 zeoite, H-mordenite zeolite, H-MCM41 zeolite, or a mixture thereof. Preferably M is selected from a group consisting of S, P, Ce, or a mixture thereof, the preferred loading of M, d is in the range of above 0.5 wt % to about 10 wt %. X is preferably F, Cl or a mixture thereof. D, the anion, is selected from a group consisting of Fxe2x88x92, Clxe2x88x92 and (HF2)xe2x88x92; the loading c, of said halogen, being preferably in the range of about 0.5 wt % to about 10 wt %.
The metallic element A is preferably selected from a group consisting of Au, Sn, or a mixture thereof, with the noble metal other than palladium B is preferably selected from the group consisting of Ru, Pt, or a mixture thereof. The mole ratios A/Pd, x, and the B/Pd, y, are both preferably in the range of about 0.001 to about 0.1. The loading percent of the metallic elements (AxByPd) b is preferably in the range of 0.5 wt % to about 7.5 wt %. The hydrophobic polymer R is preferably selected from the group consisting of polyfluorocarbons, polysiloxanes or silicone rubbers, polysulfones or a mixture thereof and the loading a, of the hydrophobic polymer is preferably in the range of about 0.05 wt % to about 5 wt %.
A number of polyfluorocarbons, polysulfones and polysiloxanes (commonly known as silicone rubbers), that are hydrophobic polymers and therefore not wetted by water or aqueous solution are known in the art. Examples of polyfluorocarbons are polyvinylidine fluoride, polyvinylidine fluoride-hexafluoropropylene copolymer, polytetrafluoroethylene, polychloretrifluoroethylene and polyethylene-tetrafluoroethylene copolymer. Examples of polysulfones are polysulfone, polyethersulfone, polyphenylsulfone and other hydrophobic polymers containing sulfur dioxide groups. Examples of polysiloxanes are polydimethylsiloxane, polymethylphenylsiloxane, polytrifluoropropylmethylsiloxane, and copolymers of dimethylsiloxane with methylphenylsiloxane, phenylvinylsiloxane or methylvinylsiloxane. Other examples of hydrophobic polymer are polysulphide rubbers. Among the above hydrophobic polymers, the most preferred are selected from the group consisting of polyvinylidine fluoride, polyethersulfone, and polydimethylsiloxane containing less than 1% vinyl groups.
In step (iv) of the process, the organic solvent for the hydrophobic polymer is selected from C6 to C8 alkanes, benzene, toluene, xylenes, dimethyl acetamide, dimethyl formamide and dimethylsulfoxide and the crosslinking agent when used, may be trimethylol propane or benzoyl peroxide or a commercial product for example SLE 5300B obtained from GE Silicones (India) Pvt. Ltd.
In an embodiment of this invention, the catalytic porous solid N is an acidic porous solid selected from the group consisting of xcex3- or xcex7-alumina, silica alumina, gallium oxide, cerium oxide, amorphous zirconia or zirconium hydroxide, thorium oxide, H-ZSM-5 zeolite, H-ZSM-11 zeolite, H-ZSM-8 zeolite, H-mordenite zeolite, H-MCM-41-zeolite, or any mixture thereof.
In an embodiment, M is selected from the group of elements selected from S, Ce, P or any mixture thereof.
In yet another embodiment (d) is in the range of from 0.5 wt % to 10 wt %.
In a further embodiment, the halaogen element X is F, Cl or a mixture thereof.
In a preferred embodiment, the anion D is selected from the group consisting of Fxe2x88x92, Clxe2x88x92 and (HF2)xe2x88x92.
In another embodiment, (c) is in the range of about 0.5 wt % to 10 wt %
In a preferred embodiment, said transition element A is selected from the group consisting of Au, Sn or a mixture thereof said noble metal element other than Pd, B is selected from Ru, Pt or a mixture thereof.
In a yet another embodiment, said A/Pd mole ratio, x is in the range of from 0.001 to 0.1.
In another preferred embodiment, said B/Pd mole ratio, y is in the range of about 0.001 to 0.1.
In another embodiment, said (b) is in the range of about 0.5 wt % to about 7.5 wt %.
The catalyst prepared by the process of this invention can be used in any catalytic process for the production of hydrogen peroxide by the reaction between hydrogen and oxygen in a liquid reaction medium comprising water, with a high conversion of hydrogen and high selectivity for hydrogen peroxide formation, even at atmospheric pressure and room temperature.
It is observed that because of the deposition of the hydrophobic polymer membrane on the catalyst, the selectivity for the hydrogen peroxide in the direct oxidation of hydrogen by oxygen to hydrogen peroxide in an aqueous medium is increased. It is also observed that both the hydrogen conversion activity and the selectivity for hydrogen peroxide formation in the direct oxidation of hydrogen by oxygen to hydrogen peroxide, of the catalyst of the invention is high due to the highly acidic nature of the catalyst. Another important finding of the invention is that the catalyst of the invention can be used for the direct oxidation of hydrogen by oxygen to hydrogen peroxide even at atmospheric pressure and room temperature with high selectivity for hydrogen peroxide formation and high hydrogen conversion.